There is a general need for pressurizing and other pumping systems which can operate reliably without substantial maintenance for long periods of time. In the past, such systems have required stable environmental conditions, the use of special and relatively expensive components and units, or the employment of special configurations for enhancing the operating life of dynamic elements. Most such pumping systems use rotating components, because reciprocating pumps inherently have greater wear and somewhat greater complexity.
The bearings used in a rotating system are illustrative of the problem of balancing cost versus reliability. Large area journal bearings, for example, are extremely long life elements if a hydrodynamic effect is established and maintained using known relationships of rotational velocity, pressure and lubricating fluid viscosity. However, assuring maintenance of these conditions typically has required a source of pressurized lubricant that is itself adequately stable and protected against temperature variations. The pump must include compensation for any leakage of lubricating fluid that may occur. Ball or needle bearings can be used, but their greater costs do not insure greater reliability or longer life.
A rotating fluid pressurizer such as a turbine pump is itself a long-life component, unless it uses dynamic seals with load bearing surfaces. The nature and requirements of the associated system with which such a pump operates may, however, present special problems. In the semiconductor fabrication industry, for example, pumps are utilized to pressurize a heat transfer fluid that heats or cools, at different times, associated semiconductor fabrication tools. These tools are ordinarily configured in a "cluster", for close proximity during the different stages of semiconductor wafer fabrication. Each tool in the cluster is separately temperature controlled, and the temperature extremes may vary within a wide range such as -40.degree. C. to +100.degree. C. The space in a facility that can be devoted to the cooling system must be as limited as possible in view of the extremely high capital costs of semiconductor fabrication equipment.
Thus, some very stringent requirements must be met by the pumps which pressurize the heat transfer fluids used with different tools. The separate temperature control channels in which each pump is employed should be of small volume and low area "footprint". Within the volume, the pumps and their driving motors must be densely arrayed. Because the capital and operating costs of the fabrication tools are so high, pumping system down time is essentially intolerable, and stable long life operation (on the order of years) is needed. Because both hot and cold fluids must be pressurized by a unit, and within a small volume, the driving systems (motors) must either be designed or modified to accept the temperature extremes, which requires both added cost and space.
The fluid flow rate in temperature control units for cluster tools usually need not be high, although a substantial pressure differential must be maintained. A regenerative turbine pump of the type having a low "specific velocity" or speed is suitable for this purpose, since it is small and has only one moving component. It can also advantageously be used in other applications, where freedom from cavitation is required.
The heat transfer fluid used in modern systems, such as with the cluster tool application must itself have special properties in order to withstand the temperature extremes to be encountered while operating over a long time span. Glycol/water mixtures previously used are now being supplanted by perfluorinated compounds, which are non-toxic and have relatively stable viscosity characteristics while also having good heat transfer properties. The perfluorinated compounds, however, are sufficiently costly to require that systems using them be virtually totally free from leakage in long term usage.